1. Field of the Invention
The present invention relates to an image display.
2. Description of Related Art
In recent years, in the field of flat panel display, the liquid crystal display has commanded a substantial share. The liquid crystal display is an image display in which a liquid crystal is interposed between two sheets of substrates made of glass or the like, for controlling light and displaying an image by changing the light transmission factor or reflection factor. Even among liquid crystal displays, an active matrix type liquid crystal display using a thin film transistor (hereinafter, abbreviated as TFT) as an active pixel for each pixel is fast in response, and has a clear image, and therefore, is currently in vogue.
For the TFT, in addition to amorphous silicon TFT (a-Si TFT) liquid crystal display which has been widely used for the conventional active matrix liquid crystal display, there is a polysilicon TFT (Poly-Si TFT) having mobility of double or more digits higher than the a-Si TFT. When the mobility of the TFT is high, it is possible to cause a large current to flow by means of the TFT, and also a circuit using the TFT is capable of operating at higher speed.
Thereby, it has become possible to integrally form a driving circuit, which has been externally mounted to the outside portion of the substrate as a driver IC in a liquid crystal display using the a-Si TFT, with a pixel TFT at the peripheral portion of the substrate. Also, it has become possible to form a circuit for driving a pixel circuit for an active matrix type light emitting diode (LED) display for displaying an image by controlling the current through a luminous element. An example of a pixel circuit of the LED display is described in FIG. 1 on page 236 of the proceedings of the 7th International Display Workshop (IDW'00).
FIG. 13 shows an example of structure of an active matrix type TFT liquid crystal display. FIG. 13 is also an example in which the driving circuit is constituted by the Poly-Si TFT, and is integrally formed with the pixel TFT at the peripheral portion of the substrate. Further, FIG. 13 shows an example of the liquid crystal display for inputting a digital image signal to display an image.
A transparent substrate 151 is one of the substrates for interposing the liquid crystal therebetween, and on a display area 156 on the upper surface of the substrate, signal lines 152 are wired in the vertical direction on the page space and scanning lines 153 are wired in the horizontal direction on the page space in the matrix shape. At the intersections between the signal lines 152 and the scanning lines 153, there are pixel TFT154 and display electrodes 155. In the upper direction of the page space of the transparent substrate 151, another sheet of transparent substrate which is not shown in the drawing is laid on top of the transparent substrate 151, and the liquid crystal is interposed therebetween to constitute the liquid crystal display. On this another sheet of transparent substrate, a transparent electrode called an opposite electrode is formed on the surface of the liquid crystal side. Between the display electrode 155 and the opposite electrode, AC voltage is applied, and the image is displayed by changing the light transmission factor and reflection factor by the effective value of the AC voltage.
Usually, to their respective signal lines 152, an analog voltage signal corresponding to a signal of an image to be displayed is supplied, in synchronization with which a pulse for switching the pixel TFT154 to a specified scanning line 153 is supplied, whereby analog voltage of the signal line 152 is supplied to the display electrodes 155 of a horizontal row. Even if the pixel TFT 154 becomes OFF, voltage supplied to the display electrode 155 is retained by means of capacity with the opposite electrode or capacity provided with other wiring. Thereafter, every time an analog signal is supplied to the signal line 152, the scanning line 153 for transmitting the pulse will be changed in turn. When supplying the pulse to all the scanning lines 153 is finished, predetermined voltage is to be supplied to each display electrode 155.
As a driving circuit for supplying such a signal line 152 as described above and a signal of the scanning line 153, at the peripheral portion of the transparent substrate 151, a scanning circuit 157 and a signal circuit 158, 159 are formed by TFT.
The scanning circuit 157 is constituted by a shift register, and has a function for generating a pulse to each output G1–G2 in turn.
The signal circuit 158, 159 is, as shown in FIG. 14, composed of: a shift register 171; a latch 172; and a DA conversion circuit 173, and has a function for distributing image data to be inputted from a data signal line DB to each output S1-S3, and a function for converting a digital signal to an analog signal.
As one of indices for performance of the image display, there is a bit number of display gradation. Assuming the bit number to be n, it is possible to change brightness of each pixel to 2n levels, and an image display having a high bit number is capable of expressing an image having a smooth change in brightness and color more accurately. The bit number of display gradation of liquid crystal displays for use with latest note personal computers and the like is frequently 6-bit or higher. This bit number of display gradation is determined by a bit number of voltage gradation of a DA conversion circuit 173 of a signal circuit.
A digital image signal inputted from the data signal line DB is stored in each of latches 172 by a pulse to be outputted from the shift register 171 in order. The digital image signals stored in the respective latches are converted into analog voltage by the DA conversion circuit 173 to be outputted to S1 to S3. Also, the signal circuit 159 is also constituted by the same circuit as shown in FIG. 14.
In order to convert voltage to be applied to a liquid crystal to AC, symmetrical voltage groups VR+ and VR− are supplied to the DA conversion circuit within the signal circuit 158 and the signal circuit 159 of FIG. 13, and voltage generated by the signal circuit 158, 159 is supplied to odd-numbered and even-numbered signal lines 152 by changing over for each horizontal period or vertical period by means of a change-over switch 160 constituted by TFT.
A circuit in the peripheral portion of the signal circuit 158, 159, the scanning circuit 157 and the like is constituted by the Poly-Si TFT, whereby the circuit can be integrally formed with each element of the display area 156. Therefore, in the liquid crystal display constituted by the Poly-Si TFT, the cost can be cut down because there is no need for the driver IC for the signal circuit and the scanning circuit which have been externally mounted on to the substrate in the liquid crystal display constituted by the a-Si TFT.
An example in which the driving circuit for the liquid crystal display is constituted by the Poly-Si TFT and is integrally formed in the peripheral portion of the display area, is described in the Extended Abstracts of the 1997 International Conference on Solid State Devices and Materials pp. 348–349 FIG. 2.
In order to provide a liquid crystal display for integrally forming a driving circuit on a substrate through the use of a Poly-Si TFT, with a display gradation performance of 6-bit or more, it is necessary to incorporate a DA conversion circuit of 6-bit or more in the signal circuit 158, 159.
In the circuit area of the DA conversion circuit incorporated in the signal circuit 158, 159, when the bit number is increased, the circuit scale increases. FIG. 15 shows a circuit diagram of a 6-bit DA conversion circuit formed through the use of both an n-channel TFT 182 and a p-channel TFT 181. Taking advantage of the characteristic property that the n-channel TFT turns ON when the gate potential is high, and turns OFF when it is low, and that the p-channel TFT turns ON when the gate potential is low, and turns OFF when it is high, voltage of gradation voltage wiring V0 to V63 is to be selected at logic voltage of 6-bit in accordance with the tournament system. In this structure, when the bit number is n, a number of the data bus wiring Dbus needs n pieces, and when the n is increased, the number of the data bus wiring is increased. When n=6, the number is 6.
When the DA conversion circuit is formed on the transparent substrate 151, however, there are the following problems. For the metallic wiring layer which can be used for the wiring, there are only two types: metallic wiring for the gate of TFT, and metallic wiring connected to the source and drain of TFT. Although it is possible to make other wiring in addition to them, it is not preferable because the cost will be increased in the manufacture. When the gradation voltage wiring V0 to V63 of the DA conversion circuit 173 is wired with one layer of metallic wiring layer in the horizontal direction on the page space, the data bus wiring Dbus to be wired in the vertical direction on the page space to intersect the metallic wiring layer is to be wired through the use of only the remaining one layer metallic wiring layer. When the bus is wired through the use of only one layer, since the mutual wiring cannot be overlapped for wiring, the width and the interval of the wiring are to be included, as they are, in the width Wx of the DA conversion circuit in the horizontal direction on the page space. Also, since the liquid crystal display has as large a substrate as a few centimeters to several tens centimeters unlike LSI, the wiring interval or the wiring width become a numerical value higher than that of the LSI by a figure or more. Under the present circumstances, it is frequently about 4 μm.
In contrast to that, the width Wx of the DA conversion circuit is restrained by a pitch (=pitch of the signal line 152) of the display electrode 155. When the signal circuits 158 and 159 are arranged above and below the display area as shown in FIG. 13, a relation of Wx≦2×Px must be satisfied. In this respect, when the signal circuit is arranged only either above or below the signal circuit, a relation of Wx≦Px must be satisfied.
Even in the case where Wx>2×Px, it is possible to connect the signal line 152 to the output S1 to S3 by preparing wiring for converting the pitch, but the number of actual signal lines 152 is generally as large as hundreds to more than thousand. After all, since the area for the wiring for converting the pitch becomes enormous, this not realistic.
In the case of, for example, a 4 inch diagonal, color VGA (Vertical 480 pixels, Horizontal 640×RGB) display, since the pitch Px of the signal line 152 is about 42 μm, the maximum value of the width Wx of the DA conversion circuit is 84 μm. When the rule of the wiring width and wiring interval of the metallic wiring is 4 μm, since six pieces of Dbus wiring need (4 μm in width+4 μm in interval)×6 pieces=48 μm, an area of 57% of the width Wx of the DA conversion circuit is occupied only by the wiring, and the width which can be used for places for arranging all the TFTs and contact holes for connecting the TFT to the wiring is limited to 36 μm corresponding to the remaining 43%. As a result, it becomes difficult to lay out the circuit.
In the liquid crystal display constituted by the a-Si TFT, since there was only a pixel TFT at a place where the TFT is formed, the n-channel TFT had only to be formed. On the other hand, in the liquid crystal display constituted by Poly-Si TFT, the driving circuit is constituted by both n-channel and p-channel in many cases. Since, however, when TFTs of both n-channel and p-channel are used, the number of processes in the manufacture is increased, the cost will be higher than when constituted by only n-channel or only p-channel. Therefore, all the driving circuits are also preferably constituted by only the n-channel or only the p-channel.
FIG. 16 shows a circuit diagram for a 6-bit DA conversion circuit constituted by only the n-channel TFT. When the conversion circuit is constituted by only the n-channel TFT 183, the TFT is capable of only performing an operation which turns ON when the gate potential is high, and turns OFF when it is low, and therefore, in addition to 6-bit logic voltage, 6-bit logic voltage of their inversion signal will be required. For this reason, in this structure, 12 pieces of data bus wiring Dbus will be required. In the case of, for example, a 4 inch diagonally, resolution VGA (Vertical 480 pixels, Horizontal 640×RGB) display, since the pitch Px of the signal line 152 is about 42 μm, the maximum value of the width Wx of the DA conversion circuit is 84 μm. When the rule of the wiring width and wiring interval of the metallic wiring is 4 μm, since six pieces of Dbus wiring will require (4 μm in width+4 μm in interval)×12 pieces=96 μm, it cannot be accommodated in the width Wx of the DA conversion circuit. Further, a place for arranging all the TFTs and contact holes for connecting the TFT to the wiring cannot be secured. Accordingly, in the present wiring rule of about 4 μm, it is exceedingly difficult to form the 6-bit DA conversion circuit.
When the pitch Px of the display electrode is enlarged in order to enlarge the width Wx of the DA conversion circuit, it becomes impossible to display a fine image. For this reason, the performance of resolution of the liquid crystal display will be degraded, and this is not preferable.
Also, in FIG. 13, there is a method for dividing the signal circuit 158 into two circuits to pile up in the vertical direction on the page space, and in the case of this method, the signal circuit width Wy of FIG. 14 is increased to twice. When the signal circuit width Wy of FIG. 14 is large, a large area which does not contribute to image displaying is to exist in the peripheral portion of the display area 156. This limits degrees of freedom of size of applied products to the display and of position for arranging the display within the applied products, which is not desirable.
Also, since piling up the signal circuit 158 in the vertical direction on the page space increases wiring to be routed within the signal circuit, structure in which width and interval of the wiring are further limited will be given. The same is applicable to the signal circuit 159.